PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 597 



operation. If a studio is equipped with them, however, thej^ might well be used. 

 Enclosed metallic arcs, such as the Pointolite and Tungsarc lamps do not have suffi- 

 cient energy in the ultraviolet to be of general use. 



Gaseous-discharge Lamps. — The gaseous-discharge lamps, particularly the mercury 

 arcs, provide the most convenient sources for ultraviolet photography. The earliest 

 form was the Cooper-Hewitt mercury- vapor lamp, which consists of an evacuated 

 glass tube containing mercury and two metal electrodes connected to a source of elec- 

 tric supply. An arc discharge is caused to pass between the electrodes by tilting the 

 tube to make and break a conducting link of mercury or by applying a high potential 

 across the electrodes. The light from the tube consists of the emission spectrum of 

 mercury vapor. The strongest line in the near ultraviolet, and the one most useful 

 photographically, is that at 3650 A. Improvements in the lamp consisted in introduc- 

 ing a considerable pressure of mercury vapor and introducing rare gases into low- 

 pressure mercury tubes, provided with electrodes of oxides of the rare earths and 

 heated by a tungsten filament. A very efficient type is the high-pressure mercury 

 lamp in which oxide-coated electrodes are used and the mercury is at a relatively high 

 pressure, the amount present being limited so that it is all vaporized before the normal 

 operating temperature is reached. The most recent types consist of a short narrow- 

 bore quartz capillary tube, and they operate at mercury- vapor pressures up to 40 atm. 

 in the open air and several hundred atmospheres when water-cooled. These capillary 

 lamps emit energy of very high intensity, but there is a strong background of continu- 

 ous spectrum in addition to the mercury lines. In addition to ordinary glass, mercury 

 lamps are made of glass having transmission farther in the ultraviolet, such as Corning 

 Corex A, Corex D, and Pj^rex. For the highest ultraviolet emission, quartz tubes are 

 used instead of glass. The lamps are of various shapes, mainly long straight tubes 

 and tubes bent into U or M form, and are manufactured by the General Electric Vapor 

 Lamp Co., Hoboken, N. J., and the Hanovia Chemical and Mfg. Co., Newark, N. J. 

 Most of these lamps are designed for general studio illumination, laboratory work, and 

 as light sources for enlargers. They can obviously be readily adapted for copying by 

 ultraviolet. In order to ensure the maximum of convenience for this type of work, 

 L. Bendikson, of the Huntington Library, San Marino, Calif., designed a quartz 

 mercury-vapor lamp in the form of a spiral, which could be placed in a metal reflector 

 round the camera lens. The sun lamps S-1 and S-2, made by the General Electric 

 Co., may be classed as mercury-arc lamps, although some light is emitted from incan- 

 descent tungsten electrodes. There are two electrodes bridged by a filament in an 

 evacuated bulb containing mercury. When a voltage is applied to the filament 

 through an appropriate transformer, an arc forms in the mercury vapor between the 

 electrodes. There is high energy emission at the 3650 A. line and a strong continuous 

 spectrum due to the filament. The bulb is of ultraviolet-transmitting Corex glass, and 

 the lamp may be used for ultraviolet photography if steps are taken to filter out the 

 visible spectrum. The lanips are not so efficient as the other mercury-discharge 

 lamps for photographic purposes. 



Other Sources. — There are many other sources which have a strong emission in the 

 ultraviolet. They include metallic arcs, such as those with electrodes of iron, electric 

 sparks, and Geissler tubes. They are of great importance for spectrography but are 

 not convenient for general ultraviolet photography. 



Filters. — FUters for ultraviolet photography are of two kinds: (1) those which 

 transmit the ultraviolet and absorb visible light, and (2) those which absorb the ultra- 

 violet and transmit visible light. In photography, filters of the first kind are used over 

 the source, or over the lens, by reflected ultraviolet. They generally consist of a 

 special glass containing nickel oxide and are of a very deep purple or opaque appear- 

 ance. Those in most common use are the Wratten No. 18A filter, which is usually 



